I just bought Bev McConnell's books from Ayrlies. I'm only 20 pages in so far, but had to stop to write this note. She said that to create a good garden you have to use a combination of these three principles:
1 - find out the spirit of the place, what does it want to be, what is sympathetic to the natural surroundings and environment.
2 - use science. Where do plants come from, what is their natural environment, how do they grow and why
3 - now use art to put these plants together. Create artistic compositions within the architecture of the landscape.
And most importantly, a garden is never finished. It is constantly evolving and changing.